Why is undo/redo not working!
If you are using English Software (Blender) under Windows 
based Operating Systems but with an AZERTY keyboard, accidentally hitting Shift+Alt keys toggles keyboard layouts, as explained here.

This apparently "disables" the Undo/Redo operators, when in fact what happens is it changes the keyboard shortcuts

Ofcourse they still work, but now you need to use Ctrl+W to Undo instead of the usuall Ctrl+Z, or use Shift+Alt to toggle back to your usual keyboard layout.

You can also disable the shortcut in the Regional Options of the control panel if you follow the steps described here

Credit goes to 3fingeredfrog for the explanation